Method and system for mapping pilot signals in multi-stream transmissions

ABSTRACT

A base station comprises a downlink transmit path comprising circuitry configured to transmit a plurality of reference signals in two or more subframes. Each subframe comprises one or more resource blocks. Each resource block comprises S OFDM symbols. Each of the S OFDM symbols comprises N subcarriers, and each subcarrier of each OFDM symbol comprises a resource element. The base station further comprises a reference signal allocator configured to allocate a first group of the plurality of reference signals to selected resource elements of a first subframe according to a reference signal pattern. The first group of the plurality of reference signals is for a first group of antenna ports. The reference signal allocator also configured to allocate a second group of the plurality of reference signals to selected resource elements of a second subframe according to the same reference signal pattern. The second group of the plurality of reference signals is for a second group of antenna ports different from the first group of antenna ports.

CROSS-REFERENCE TO RELATED APPLICATION(S) AND CLAIM OF PRIORITY

This application is a continuation of U.S. Non-Provisional patentapplication Ser. No. 14/571,162 filed Dec. 15, 2014 and entitled “METHODAND SYSTEM FOR MAPPING PILOT SIGNALS IN MULTI-STREAM TRANSMISSIONS,”which is a continuation of U.S. Non-Provisional patent application Ser.No. 12/709,399 filed Feb. 19, 2010 and entitled “METHOD AND SYSTEM FORMAPPING PILOT SIGNALS IN MULTI-STREAM TRANSMISSIONS,” which claimspriority to U.S. Provisional Patent Application No. 61/210,290 filedMar. 17, 2009 and entitled “PILOT MAPPING METHODS FOR MULTI-STREAMTRANSMISSIONS IN OFDM SYSTEMS.” The content of the above-identifiedpatent documents is hereby incorporated by reference.

TECHNICAL FIELD

The present application relates generally to wireless communicationsand, more specifically, to a method and system for reference signal (RS)pattern design.

BACKGROUND

In 3^(rd) Generation Partnership Project Long Term Evolution (3GPP LTE),Orthogonal Frequency Division Multiplexing (OFDM) is adopted as adownlink (DL) transmission scheme.

SUMMARY

A base station comprises a downlink transmit path comprising circuitryconfigured to transmit a plurality of reference signals in two or moresubframes. Each subframe comprises one or more resource blocks. Eachresource block comprises S OFDM symbols. Each of the S OFDM symbolscomprises N subcarriers, and each subcarrier of each OFDM symbolcomprises a resource element. The base station further comprises areference signal allocator configured to allocate a first group of theplurality of reference signals to selected resource elements of a firstsubframe according to a reference signal pattern. The first group of theplurality of reference signals is for a first group of antenna ports.The reference signal allocator also configured to allocate a secondgroup of the plurality of reference signals to selected resourceelements of a second subframe according to the same reference signalpattern. The second group of the plurality of reference signals is for asecond group of antenna ports different from the first group of antennaports.

A subscriber station comprises a downlink receive path comprisingcircuitry configured to receive a plurality of reference signals in twoor more subframes. Each subframe comprises one or more resource blocks.Each resource block comprises S OFDM symbols. Each of the S OFDM symbolscomprises N subcarriers, and each subcarrier of each OFDM symbolcomprises a resource element. The subscriber station further comprises areference signal receiver configured to receive a first group of theplurality of reference signals allocated to selected resource elementsof a first subframe according to a reference signal pattern. The firstgroup of the plurality of reference signals is for a first group ofantenna ports. The reference signal receiver is also configured toreceive a second group of the plurality of reference signals allocatedto selected resource elements of a second subframe according to the samereference signal pattern. The second group of the plurality of referencesignals is for a second group of antenna ports different from the firstgroup of antenna ports.

A base station comprises a downlink transmit path comprising circuitryconfigured to transmit a plurality of reference signals in one or moreresource blocks. Each resource block comprises S OFDM symbols. Each ofthe S OFDM symbols comprises N subcarriers, and each subcarrier of eachOFDM symbol comprises a resource element. The base station furthercomprises a reference signal allocator configured to allocate a firstgroup of resource elements for reference signal mapping. The first groupof resource elements has a first group of reference signals. Each of thefirst group of reference signals is assigned for a respective layerselected from a first group of layers. All of the layers assigned to thefirst group of reference signals are mapped to a first codeword. Thereference signal allocator is also configured to allocate a second groupof resource elements for reference signal mapping. The second group ofresource elements has a second group of reference signals. Each of thesecond group of reference signals is assigned for a respective layerselected from a second group of layers. All of the layers assigned tothe second group of reference signals are mapped to a second codeword,the second codeword being different from the first codeword.

A subscriber station comprises a downlink receive path comprisingcircuitry configured to receive a plurality of reference signals in oneor more resource blocks. Each resource block comprises S OFDM symbols.Each of the S OFDM symbols comprising N subcarriers, and each subcarrierof each OFDM symbol comprises a resource element. The subscriber stationfurther comprises a reference signal receiver configured to receive afirst group of resource elements allocated for reference signal mapping.The first group of resource elements has a first group of referencesignals. Each of the first group of reference signals is assigned for arespective layer selected from a first group of layers. All of thelayers assigned to the first group of reference signals are mapped to afirst codeword. The reference signal receiver is also configured toreceive a second group of resource elements allocated for referencesignal mapping. The second group of resource elements has a second groupof reference signals. Each of the second group of reference signals isassigned for a respective layer selected from a second group of layers.All of the layers assigned to the second group of reference signals aremapped to a second codeword, the second codeword being different fromthe first codeword.

A base station comprises a downlink transmit path comprising circuitryconfigured to transmit a plurality of reference signals in a resourceblock. The resource block comprises S OFDM symbols. Each of the S OFDMsymbols comprises N subcarriers, and each subcarrier of each OFDM symbolcomprises a resource element. The base station further comprises areference signal allocator configured to allocate a number of resourceelements of the resource block for transmitting reference signalscorresponding to a first antenna port, and to adjust the number ofresource elements used for transmitting reference signals correspondingto the first antenna port based at least partly upon a total number ofantenna ports in the resource block.

A subscriber station comprises a downlink receive path comprisingcircuitry configured to receive a plurality of reference signals in oneor more resource blocks. Each resource block comprises S OFDM symbols.Each of the S OFDM symbols comprises N subcarriers, and each subcarrierof each OFDM symbol comprises a resource element. The subscriber stationfurther comprises a reference signal receiver configured to receive anumber of resource elements of the resource block allocated fortransmitting reference signals corresponding to a first antenna port.The number of resource elements allocated for transmitting referencesignals corresponding to the first antenna port is based at least partlyupon a total number of antenna ports in the resource block.

A base station comprises a downlink transmit path comprising circuitryconfigured to transmit a plurality of reference signals in two or moresubframes. Each subframe comprises one or more resource blocks. Eachresource block comprises S OFDM symbols. Each of the S OFDM symbolscomprises N subcarriers, and each subcarrier of each OFDM symbolcomprises a resource element. The base station further comprises areference signal allocator configured to allocate a plurality ofresource elements of the resource block for transmitting referencesignals corresponding to one or more antenna ports and to group theplurality of resource elements into one or more groups of resourceelements. Each group of resource elements has a same number of resourceelements. The reference signals within each of the one or more groups ofresource elements are multiplexed using a code division multiplexing(CDM), and the number of resource elements in each of the one or moregroups of resource elements is based at least partly upon a rank of theresource block.

A subscriber station comprises a downlink receive path comprisingcircuitry configured to receive a plurality of reference signals in oneor more resource blocks. Each resource block comprises S OFDM symbols.Each of the S OFDM symbols comprises N subcarriers, and each subcarrierof each OFDM symbol comprises a resource element. The subscriber stationfurther comprises a reference signal receiver configured to receive aplurality of resource elements of the resource block for transmittingreference signals corresponding to one or more antenna ports. Theplurality of resource elements are grouped into one or more groups ofresource elements with each group of resource elements having a samenumber of resource elements. The reference signals within each of theone or more groups of resource elements are multiplexed using a codedivision multiplexing (CDM), and the number of resource elements in eachof the one or more groups of resource elements is based at least partlyupon a rank of the resource block.

Before undertaking the DETAILED DESCRIPTION below, it may beadvantageous to set forth definitions of certain words and phrases usedthroughout this patent document: the terms “include” and “comprise,” aswell as derivatives thereof, mean inclusion without limitation; the term“or,” is inclusive, meaning and/or; the phrases “associated with” and“associated therewith,” as well as derivatives thereof, may mean toinclude, be included within, interconnect with, contain, be containedwithin, connect to or with, couple to or with, be communicable with,cooperate with, interleave, juxtapose, be proximate to, be bound to orwith, have, have a property of, or the like; and the term “controller”means any device, system or part thereof that controls at least oneoperation, such a device may be implemented in hardware, firmware orsoftware, or some combination of at least two of the same. It should benoted that the functionality associated with any particular controllermay be centralized or distributed, whether locally or remotely.Definitions for certain words and phrases are provided throughout thispatent document, those of ordinary skill in the art should understandthat in many, if not most instances, such definitions apply to prior, aswell as future uses of such defined words and phrases.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsadvantages, reference is now made to the following description taken inconjunction with the accompanying drawings, in which like referencenumerals represent like parts:

FIG. 1 illustrates an exemplary wireless network that transmits messagesin the uplink according to the principles of the present disclosure;

FIG. 2 is a high-level diagram of an OFDMA transmitter according to oneembodiment of the disclosure;

FIG. 3 is a high-level diagram of an OFDMA receiver according to oneembodiment of the disclosure;

FIG. 4 illustrates channel quality information (CQI) reference signalpatterns according to an embodiment of the disclosure;

FIG. 5 illustrates CQI reference signal patterns that provide pilotsignals for up to 4 transmit antenna-port channels in a resource blockaccording to an embodiment of the disclosure;

FIG. 6 illustrates alternating sets of antenna ports for which a CQIreference signal pattern provides pilots according to an embodiment ofthe disclosure;

FIG. 7 illustrates user equipment (UE)-specific demodulation referencesignal (DM RS) mapping patterns according to an embodiment of thedisclosure;

FIG. 8 illustrates DM RS mapping patterns used for downlinktransmission, where a subset of the DM RS resource elements providespilots for multiple streams (or layers) according to an embodiment ofthe disclosure;

FIG. 9 illustrates a DM RS mapping pattern in which the DM RS densityper stream is reduced according to an embodiment of the disclosure;

FIG. 10 illustrates DM RS mapping patterns in which code divisionmultiplexing (CDM) is applied to multiplex more pilot signals when thenumber of streams (or layers) multiplexed in a resource block reaches acertain limit according to an embodiment of the disclosure;

FIG. 11 is a table 1100 showing the mapping of code division multiplexed(CDMed) pilot signals according to an embodiment of the disclosure;

FIG. 12 is a table showing the mapping of CDMed pilot signals accordingto another embodiment of the disclosure;

FIG. 13 is a table showing the mapping of CDMed pilot signals accordingto a further embodiment of the disclosure;

FIG. 14 illustrates a method of operating a subscriber station accordingto an embodiment of the disclosure; and

FIG. 15 illustrates another method of operating a subscriber stationaccording to an embodiment of the disclosure.

DETAILED DESCRIPTION

FIGS. 1 through 15, discussed below, and the various embodiments used todescribe the principles of the present disclosure in this patentdocument are by way of illustration only and should not be construed inany way to limit the scope of the disclosure. Those skilled in the artwill understand that the principles of the present disclosure may beimplemented in any suitably arranged wireless communication system.

With regard to the following description, it is noted that the LTE term“node B” is another term for “base station” used below. Also, the LTEterm “user equipment” or “UE” is another term for “subscriber station”used below.

FIG. 1 illustrates exemplary wireless network 100, which transmitsmessages according to the principles of the present disclosure. In theillustrated embodiment, wireless network 100 includes base station (BS)101, base station (BS) 102, base station (BS) 103, and other similarbase stations (not shown).

Base station 101 is in communication with Internet 130 or a similarIP-based network (not shown).

Base station 102 provides wireless broadband access to Internet 130 to afirst plurality of subscriber stations within coverage area 120 of basestation 102. The first plurality of subscriber stations includessubscriber station 111, which may be located in a small business (SB),subscriber station 112, which may be located in an enterprise (E),subscriber station 113, which may be located in a WiFi hotspot (HS),subscriber station 114, which may be located in a first residence (R),subscriber station 115, which may be located in a second residence (R),and subscriber station 116, which may be a mobile device (M), such as acell phone, a wireless laptop, a wireless PDA, or the like.

Base station 103 provides wireless broadband access to Internet 130 to asecond plurality of subscriber stations within coverage area 125 of basestation 103. The second plurality of subscriber stations includessubscriber station 115 and subscriber station 116. In an exemplaryembodiment, base stations 101-103 may communicate with each other andwith subscriber stations 111-116 using OFDM or OFDMA techniques.

While only six subscriber stations are depicted in FIG. 1, it isunderstood that wireless network 100 may provide wireless broadbandaccess to additional subscriber stations. It is noted that subscriberstation 115 and subscriber station 116 are located on the edges of bothcoverage area 120 and coverage area 125. Subscriber station 115 andsubscriber station 116 each communicate with both base station 102 andbase station 103 and may be said to be operating in handoff mode, asknown to those of skill in the art.

Subscriber stations 111-116 may access voice, data, video, videoconferencing, and/or other broadband services via Internet 130. In anexemplary embodiment, one or more of subscriber stations 111-116 may beassociated with an access point (AP) of a WiFi WLAN. Subscriber station116 may be any of a number of mobile devices, including awireless-enabled laptop computer, personal data assistant, notebook,handheld device, or other wireless-enabled device. Subscriber stations114 and 115 may be, for example, a wireless-enabled personal computer(PC), a laptop computer, a gateway, or another device.

FIG. 2 is a high-level diagram of an orthogonal frequency divisionmultiple access (OFDMA) transmit path 200. FIG. 3 is a high-leveldiagram of an orthogonal frequency division multiple access (OFDMA)receive path 300. In FIGS. 2 and 3, the OFDMA transmit path 200 isimplemented in base station (BS) 102 and the OFDMA receive path 300 isimplemented in subscriber station (SS) 116 for the purposes ofillustration and explanation only. However, it will be understood bythose skilled in the art that the OFDMA receive path 300 may also beimplemented in BS 102 and the OFDMA transmit path 200 may be implementedin SS 116.

The transmit path 200 in BS 102 comprises a channel coding andmodulation block 205, a serial-to-parallel (S-to-P) block 210, a Size NInverse Fast Fourier Transform (IFFT) block 215, a parallel-to-serial(P-to-S) block 220, an add cyclic prefix block 225, an up-converter (UC)230, a reference signal multiplexer 290, and a reference signalallocator 295.

The receive path 300 in SS 116 comprises a down-converter (DC) 255, aremove cyclic prefix block 260, a serial-to-parallel (S-to-P) block 265,a Size N Fast Fourier Transform (FFT) block 270, a parallel-to-serial(P-to-S) block 275, and a channel decoding and demodulation block 280.

At least some of the components in FIGS. 2 and 3 may be implemented insoftware while other components may be implemented by configurablehardware or a mixture of software and configurable hardware. Inparticular, it is noted that the FFT blocks and the IFFT blocksdescribed in the present disclosure document may be implemented asconfigurable software algorithms, where the value of Size N may bemodified according to the implementation.

Furthermore, although the present disclosure is directed to anembodiment that implements the Fast Fourier Transform and the InverseFast Fourier Transform, this is by way of illustration only and shouldnot be construed to limit the scope of the disclosure. It will beappreciated that in an alternate embodiment of the disclosure, the FastFourier Transform functions and the Inverse Fast Fourier Transformfunctions may easily be replaced by Discrete Fourier Transform (DFT)functions and Inverse Discrete Fourier Transform (IDFT) functions,respectively. It will be appreciated that for DFT and IDFT functions,the value of the N variable may be any integer number (i.e., 1, 2, 3, 4,etc.), while for FFT and IFFT functions, the value of the N variable maybe any integer number that is a power of two (i.e., 1, 2, 4, 8, 16,etc.).

In BS 102, channel coding and modulation block 205 receives a set ofinformation bits, applies coding (e.g., Turbo coding) and modulates(e.g., QPSK, QAM) the input bits to produce a sequence offrequency-domain modulation symbols. Serial-to-parallel block 210converts (i.e., de-multiplexes) the serial modulated symbols to paralleldata to produce N parallel symbol streams where N is the IFFT/FFT sizeused in BS 102 and SS 116. Size N IFFT block 215 then performs an IFFToperation on the N parallel symbol streams to produce time-domain outputsignals. Parallel-to-serial block 220 converts (i.e., multiplexes) theparallel time-domain output symbols from Size N IFFT block 215 toproduce a serial time-domain signal. Add cyclic prefix block 225 theninserts a cyclic prefix to the time-domain signal. Finally, up-converter230 modulates (i.e., up-converts) the output of add cyclic prefix block225 to RF frequency for transmission via a wireless channel. The signalmay also be filtered at baseband before conversion to RF frequency. Insome embodiments, reference signal multiplexer 290 is operable tomultiplex the reference signals using code division multiplexing (CDM)or time/frequency division multiplexing (TFDM). Reference signalallocator 295 is operable to dynamically allocate reference signals inan OFDM signal in accordance with the methods and system disclosed inthe present disclosure.

The transmitted RF signal arrives at SS 116 after passing through thewireless channel and reverse operations performed at BS 102.Down-converter 255 down-converts the received signal to basebandfrequency and remove cyclic prefix block 260 removes the cyclic prefixto produce the serial time-domain baseband signal. Serial-to-parallelblock 265 converts the time-domain baseband signal to parallel timedomain signals. Size N FFT block 270 then performs an FFT algorithm toproduce N parallel frequency-domain signals. Parallel-to-serial block275 converts the parallel frequency-domain signals to a sequence ofmodulated data symbols. Channel decoding and demodulation block 280demodulates and then decodes the modulated symbols to recover theoriginal input data stream.

Each of base stations 101-103 may implement a transmit path that isanalogous to transmitting in the downlink to subscriber stations 111-116and may implement a receive path that is analogous to receiving in theuplink from subscriber stations 111-116. Similarly, each one ofsubscriber stations 111-116 may implement a transmit path correspondingto the architecture for transmitting in the uplink to base stations101-103 and may implement a receive path corresponding to thearchitecture for receiving in the downlink from base stations 101-103.The present disclosure describes a method and system for referencesignal (RS) pattern design.

The total bandwidth in an OFDM system is divided into narrowbandfrequency units called subcarriers. The number of subcarriers is equalto the FFT/IFFT size N used in the system. In general, the number ofsubcarriers used for data is less than N because some subcarriers at theedge of the frequency spectrum are reserved as guard subcarriers. Ingeneral, no information is transmitted on guard subcarriers.

The transmitted signal in each downlink (DL) slot of a resource block isdescribed by a resource grid of N_(RB) ^(DL)N_(sc) ^(RB) subcarriers andN_(symb) ^(DL) OFDM symbols. The quantity N_(RB) ^(DL) depends on thedownlink transmission bandwidth configured in the cell and fulfillsN_(RB) ^(min,DL)≦N_(RB) ^(DL)≦N_(RB) ^(max,DL), where N_(RB) ^(min,DL)and N_(RB) ^(max,DL) are the smallest and largest downlink bandwidth,respectively, supported. In some embodiments, subcarriers are consideredthe smallest elements that are capable of being modulated.

In case of multi-antenna transmission, there is one resource griddefined per antenna port. Each element in the resource grid for antennaport p is called a resource element (RE) and is uniquely identified bythe index pair (k,l) in a slot where k=0, . . . , N_(RB) ^(DL)N_(sc)^(RB)−1 and l=0, . . . , N_(symb) ^(DL)−1 are the indices in thefrequency and time domains, respectively. Resource element (k,l) onantenna port p corresponds to the complex value a_(k,l) ^((p)). If thereis no risk for confusion or no particular antenna port is specified, theindex p may be dropped.

In LTE, DL reference signals (RSs) are used for two purposes. First, UEsmeasure channel quality information (CQI), rank information (RI) andprecoder matrix information (PMI) using DL RSs. Second, each UEdemodulates the DL transmission signal intended for itself using the DLRSs. In addition, DL RSs are divided into three categories:cell-specific RSs, multi-media broadcast over a single frequency network(MBSFN) RSs, and UE-specific RSs or dedicated RSs (DRSs).

Cell-specific reference signals (or common reference signals: CRSs) aretransmitted in all downlink subframes in a cell supporting non-MB SFNtransmission. If a subframe is used for transmission with MBSFN, onlythe first a few (0, 1 or 2) OFDM symbols in a subframe can be used fortransmission of cell-specific reference symbols. The notation R_(p) isused to denote a resource element used for reference signal transmissionon antenna port p.

UE-specific reference signals (or dedicated RS: DRS) are supported forsingle-antenna-port transmission of Physical Downlink Shared Channel(PDSCH) and are transmitted on antenna port 5. The UE is informed byhigher layers whether the UE-specific reference signal is present and isa valid phase reference for PDSCH demodulation or not. UE-specificreference signals are transmitted only on the resource blocks upon whichthe corresponding PDSCH is mapped.

The time resources of an LTE system are partitioned into 10 msec frames,and each frame is further partitioned into 10 subframes of one msecduration each. A subframe is divided into two time slots, each of whichspans 0.5 msec. A subframe is partitioned in the frequency domain intomultiple resource blocks (RBs), where an RB is composed of 12subcarriers.

In an embodiment of the disclosure, a CQI reference signal (CQI RS)mapping pattern is defined as a set of resource elements (REs) in oneresource block (RB) spanning two slots (or one subframe), where thepattern repeats every RB in a subset or in the set of RBs in the systembandwidth.

In particular embodiments, CQI RS REs reside in only one slot or in bothslots in an RB in one subframe.

FIG. 4 illustrates CQI reference signal patterns according to anembodiment of the disclosure.

In FIG. 4, cross-hatched resource elements 401 indicate demodulation(DM) RS REs. Resource block 403 illustrates an example where CQI RS REsreside in both slots in an RB. As shown in resource block 403, a CQI RSpattern appears in OFDM symbol 5 in the even-numbered slots and OFDMsymbol 3 in the odd-numbered slots. Of course, one of ordinary skill inthe art would recognize that DM RS mapping patterns similar to thepattern shown in resource block 403 can be constructed by choosingdifferent OFDM symbols for the DM RS REs. Resource block 405 illustratesan example where CQI RS REs reside in only one slot in an RB. Inresource block 405, the CQI RS pattern has CQI RS REs only in one slot,which is OFDM symbol 3 in the odd-numbered slots.

In an embodiment of this disclosure, a CQI RS mapping pattern isprovided for estimating channel state information (CSI) at the receiverside for multiple Tx antenna-port channels, where CSI includes channelquality information (CQI), rank information (RI), and precoding matrixinformation (PMI), and channel direction information (CDI), and soforth.

FIG. 5 illustrates CQI reference signal patterns that provide pilotsignals for up to 4 transmit antenna-port channels in a resource blockaccording to an embodiment of the disclosure.

Each set of CQI RS REs labeled with number i carries the pilot signalsfor antenna port i , where i=0,1,2,3. In an embodiment of thisdisclosure, one CQI RS pattern is used for different sets of antennaports in different subframes. For example, in resource blocks 501 and503, the CQI RS pattern is used for sending pilots for antenna ports 0,1, 2 and 3 in some subframes, while in other subframes as shown inresource blocks 505 and 507, the same CQI RS pattern is used for antennaports 4, 5, 6 and 7, where (4,5,6,7) is mapped to RS REs labeled with(0,1,2,3), respectively.

In an embodiment of this disclosure, the CQI subframes (i.e., thesubframes where CQI RSs are transmitted) can be either periodically set(e.g., every 5 subframes) or aperiodically set. The period and offsetfor the CQI RS subframes can be set by broadcast signaling or by animplicit function of the cell-id and subframe number. In a particularembodiment, the remainder of the division of the cell-id by the periodbecomes the CQI RS subframe offset. For example, the CQI RS subframeperiod is set to be 5 subframes by a broadcasted value. In this case, acell with cell-id 2 has CQI RS in subframes 2 and 7, while a cell withcell-id 3 has CQI RS in subframes in 3 and 8.

FIG. 6 illustrates alternating sets of antenna ports for which a CQIreference signal pattern provides pilots according to an embodiment ofthe disclosure.

In some embodiments, the sets of antenna ports for which a CQI RSpattern provides pilots alternate as the subframes progress in time. Forexample, in a first CQI RS subframe, pilots for ports 0 and 1 aretransmitted; in a second CQI RS subframe, pilots for ports 2 and 3 aretransmitted; in a third CQI RS subframe, pilots for ports 4 and 5 aretransmitted; in a fourth CQI RS subframe, pilots for ports 6 and 7 aretransmitted; in a fifth CQI RS subframe, pilots for ports 0 and 1 aretransmitted again, and so forth. In another example shown in FIG. 6, ina first CQI RS subframe (subframe #2), pilots for ports 0, 1, 2 and 3are transmitted using the CQI RS pattern in resource block 501. In asecond such subframe (subframe #3), pilots for ports 4, 5, 6 and 7 aretransmitted. In a third such subframe (subframe #7), pilots for ports 0,1, 2, 3 are transmitted; in a fourth such subframe (subframe #8), pilotsfor ports 4, 5, 6, 7, are transmitted, and so forth.

In further embodiments, each group of CQI subframes that contains CQI RSREs for a set of antenna ports can be either periodically set (e.g.,every 5 subframes) or aperiodically set.

In one embodiment, the periods and offsets for all the groups of CQI RSsubframes can be set by broadcast signaling or by an implicit functionof the cell-id and subframe number.

In another embodiment, the period and offset for one group (denoted bythe first group) of CQI RS subframes can be set by broadcast signalingor by an implicit function of the cell-id and subframe number, while theperiods and offsets for the other groups of CQI RS subframes areimplicitly indicated by the signaling.

In a particular embodiment, the periods of all the other groups of CQIRS subframes are the same as the first group. The offset of the secondgroup is one greater than that of the first group, and the offset of thethird group is two greater than that of the first group, and so on. Whenthe offset value calculated by this method is greater than the period,the subframe offset is re-calculated as the one obtained aftersubtracting the period from the old offset value.

For example, as shown in FIG. 6, the CQI RS subframe period is set to be5 subframes by a broadcasted value. In this case, a cell with cell-id 2has CQI RS in subframes 2 and 7 for sending CQI RS for antenna ports 0,1, 2 and 3. In addition, according to the example offset rule for thesecond group, this cell has CQI RS for antenna ports 4, 5, 6 and 7 insubframes 3 and 8.

In other embodiments, when a CQI RS subframe determined by a period andan offset happens to be a primary synchronization signal (PSS) subframeor a secondary synchronization signal (SSS) subframe, the CQI RS in thePSS/SSS subframe may be dropped, transmitted right after the PSS/SSSsubframe, or transmitted in a subset of RBs in the PSS/SSS subframewhere PSS/SSS related signals are not transmitted.

FIG. 7 illustrates UE-specific demodulation reference signal (DM RS)mapping patterns according to an embodiment of the disclosure.

A UE-specific demodulation reference signal (DM RS) mapping patternrefers to a set of resource elements (REs) in one resource block (RB),where the pattern repeats in all the assigned RBs in a transmission. Theset of RBs can be assigned for transmission to a single UE or multipleUEs. In FIG. 7, cross-hatched resource elements 701 indicate DM RS REs.In resource block 703, the same DM RS mapping pattern is applied to OFDMsymbols 5 and 6 of both slots. Of course, one of ordinary skill in theart would recognize that DM RS mapping patterns similar to the patternshown in resource block 703 can be constructed by choosing differentOFDM symbols for the DM RS REs. In resource block 705, the same DM RSmapping pattern is applied but to different OFDM symbols from resourceblock 703. In resource block 705, the DM RS mapping pattern is appliedto OFDM symbols 2 and 3 in the odd-numbered slots.

FIG. 8 illustrates DM RS mapping patterns used for downlinktransmission, where a subset of the DM RS resource elements providespilots for multiple streams (or layers) according to an embodiment ofthe disclosure.

In resource block 801 and 802, each set of DM RS REs labeled with numberi carries the pilot signals for stream i, where i=0,1,2,3. As shown, theDM RS pattern in resource block 801 can be used to provide pilot signalsfor up to 4 multiplexed streams (or layers) in an RB while the DM RSpattern in resource block 803 can be used to provide pilot signals forone stream. In some embodiments, when the number of streams (or layers)is n≦4, DM RS REs labeled with number up to n−1 are used to carry pilotsignals, while the others are used to carry data symbols. For example,with transmission rank 1 or when only one stream is transmitted to a UE,only the DM RS REs labeled with 0 carry the pilot signals for the UE,while the other DM RS REs labeled with 1, 2, 3 may carry data symbols.

FIG. 9 illustrates a DM RS mapping pattern in which the DM RS densityper stream is reduced according to an embodiment of the disclosure.

In further embodiments of the disclosure, the DM RS mapping pattern iskept the same with different number of transmitted streams (or layers),while the per-stream RS density may decrease or be diluted when thetotal number of streams (or layers) multiplexed in an RB reaches acertain limit. For example, the DM RS pattern of resource block 801 isused to provide up to 4 pilot signals for each of 4 streams (or layers).For a higher number of total transmitted streams, the DM RS density perstream is reduced as shown in the DM RS pattern of resource block 901.In resource block 901, the number of RS REs per stream is 2 which ishalf the density of the DM RS pattern of resource block 801.

Of course, one of ordinary skill in the art would recognize that a totalnumber of resource elements allocated for a particular antenna port in aresource block also may be increased or decreased as described aboveaccording to a total number of antenna ports in the resource block asthe total number of streams or layers also represent the total number ofantenna ports in the resource block.

FIG. 10 illustrates DM RS mapping patterns in which code divisionmultiplexing (CDM) is applied to multiplex more pilot signals when thenumber of streams multiplexed in a resource block reaches a certainlimit according to an embodiment of the disclosure.

In another embodiment of the disclosure, the DM RS mapping pattern iskept the same with different number of transmitted streams, while codedivision multiplexing (CDM) is applied to multiplex more pilot signalswhen the number of streams multiplexed in an RB reaches a certain limit.In particular, the DM RS REs in a DM RS mapping pattern are partitionedinto multiple (e.g., two) groups, where the RS REs in one group has astaggered pattern in the time-frequency grid.

In a particular embodiment, when an even number (2n) of streams aremultiplexed in an RB, the streams are partitioned into two groups of theequal number (n) of streams. Then, n Walsh covers are assigned to the nstreams within each group. When the DM RS REs in a DM RS mapping patternalso are partitioned into the same number of groups (i.e., two groups),the n pilots for each group of streams are mapped onto each set of DM RSREs with their Walsh covers applied.

When an odd number (2n+1) of streams are multiplexed in an RB, thestreams are partitioned into two groups of size n and n+1, where thestreams that are to be decoded earlier at the receiver (e.g., successiveinterference cancellation: SIC receiver) are assigned to the group ofsmaller size, while the other streams are assigned to the group oflarger size. The DM RS REs in a DM RS pattern also are partitioned intotwo sets. Then, n and n+1 Walsh covers are assigned to the streams inthe two groups, respectively, and the pilots for each group of streamsare Walsh-covered and mapped onto each set of DM RS REs.

In one example, the DM RS pattern of resource block 801 is used forproviding up to 4 pilot signals for each of 4 streams. For higher numberof transmitted streams, CDM is applied on each group of four RS REs(i.e., spreading factor=4) as shown in the DM RS pattern of resourceblock 1001. In this particular example, there are four Walsh covers oflength 4: W0[1 1 1 1], W1=[1 −1 1 −1], W2=[1 −1 −1 1] and W3=[−1 1 1−1]. One Walsh cover is given for each stream. Then, the transmitterapplies each stream's Walsh cover on a pilot symbol of the stream, whichgives a length-4 sequence. This length 4 sequence is mapped onto DM RSREs labeled with either (A₁, A₂, A₃, A₄) or (B₁, B₂, B₃, B₄).

FIG. 11 is a table 1100 showing the mapping of CDMed pilot signalsaccording to an embodiment of the disclosure.

In a specific embodiment, a stream with a smaller index is to be decodedno later than another stream with a larger index at the receiver side.As shown in table 1100, when 5 streams are multiplexed on DM RS REs (A₁,A₂, A₃, A₄), antenna port 0 transmits a pilot signal W0=[1 1 1 1] whileantenna port 1 transmits a pilot signal W1=[1 −1 1 −1].

FIG. 12 is a table 1200 showing the mapping of CDMed pilot signalsaccording to another embodiment of the disclosure.

In this specific example, table 1200 summarizes the DM RS pattern ofresource block 1001 with length-2 Walsh covers (or with spreadingfactor=2) W0[1 1] and W1=[1 −1]. As shown in table 1200, when 5 streamsare multiplexed on DM RS REs (A₁, A₂, A₃, A₄), antenna ports 0 and 1transmit a pilot signal W0=[1 1].

FIG. 13 is a table 1300 showing the mapping of CDMed pilot signalsaccording to a further embodiment of the disclosure.

In this embodiment, CDM is applied on the DM RS pattern when there is asmaller number of streams as well. In a specific example, DM RS REs in aDM RS pattern are partitioned into multiple (e.g., two) groups, wheretwo adjacent DM RS REs in a group are paired for CDM Walsh covering withlength-2 Walsh covers. The RS REs in one group have a staggered patternin the time-frequency grid. For example, the DM RS pattern of resourceblock 1003 with length-2 Walsh code is used when the number of streamsis less than 5. As shown in table 1300, when 4 streams are multiplexedon DM RS REs (A₁, A₂), antenna port 0 transmits a pilot signal W0=[1 1]while antenna port 1 transmits a pilot signal W1=[1 −1].

FIG. 14 illustrates a method of operating a subscriber station accordingto an embodiment of the disclosure.

Method 1400 includes receiving, by way of a downlink receive path, aplurality of pilot reference signals in one or more resource blocks(Block 1401). Each resource block comprises S OFDM symbols, each of theS OFDM symbols comprises N subcarriers, and each subcarrier of each OFDMsymbol comprises a resource element. Method 1400 also includesreceiving, by way of a reference signal receiver, a first group of theplurality of pilot reference signals allocated to selected resourceelements of a first resource block of a first channel qualityinformation (CQI) reference signal subframe according to a referencesignal pattern (Block 1403). The first group of the plurality of pilotreference signals is for a first group of antenna ports. Method 1400further comprises receiving, by way of the reference signal receiver, asecond group of the plurality of pilot reference signals allocated toselected resource elements of a second resource block of a second CQIreference signal subframe according to the same reference signal pattern(Block 1405). The second group of the plurality of pilot referencesignals is for a second group of antenna ports different from the firstgroup of antenna ports.

FIG. 15 illustrates another method of operating a subscriber stationaccording to an embodiment of the disclosure.

Method 1500 includes receiving, by way of a downlink receive path, aplurality of reference signals in one or more resource blocks (Block1501). Each resource block comprises S OFDM symbols, each of the S OFDMsymbols comprises N subcarriers, and each subcarrier of each OFDM symbolcomprises a resource element. Method 1500 also includes receiving, byway of a reference signal receiver, the plurality of reference signalsto allocated selected resource elements of a resource block according toa reference signal pattern. The plurality of reference signalscorresponds to one or more antenna ports. The plurality of referencesignals are partitioned into two or more groups of reference signals.One or more streams corresponding to the one or more antenna ports arepartitioned into a same number of groups of streams. A Walsh cover isapplied to each stream within each group of streams. For each pilotreference signal corresponding to a group of streams, the pilotreference signal is mapped onto a corresponding group of referencesignals with the Walsh cover applied (Block 1503).

Although the present disclosure has been described with an exemplaryembodiment, various changes and modifications may be suggested to oneskilled in the art. It is intended that the present disclosure encompasssuch changes and modifications as fall within the scope of the appendedclaims.

What is claimed is:
 1. A base station, comprising: a downlink transmitpath comprising circuitry configured to transmit at least one referencesignal in one or more subframes, each subframe comprising two slots,each slot comprising S OFDM symbols, each of the S OFDM symbolscomprising N resource elements; and a reference signal controllerconfigured to: apply one orthogonal code of two orthogonal codes to anantenna port of a first set of antenna ports, if the number of layers isless than or equal to a predetermined number of layers, and apply oneorthogonal code of four orthogonal codes to an antenna port of a secondset of antenna ports, if the number of layers is greater than thepredetermined number of layers, wherein total number of antenna ports isbased on the number of layers.
 2. A base station in accordance withclaim 1, wherein one antenna port comprises 4 resource elements.
 3. Abase station in accordance with claim 1, wherein resource elements forthe at least one antenna port are placed on 6th and 7th OFDM symbols inthe each slot.
 4. A base station in accordance with claim 1, wherein afirst antenna port within the first set is mapped on a first 4 resourceelements with a first orthogonal code, a second antenna port within thefirst set is mapped on a first 4 resource elements with a secondorthogonal code, a third antenna port within the first set is mapped ona second 4 resource elements with the first orthogonal code, and afourth antenna port within the first set is mapped on a second 4resource elements with the second orthogonal code.
 5. A base station inaccordance with claim 1, wherein the predetermined number of layers is4.
 6. A base station in accordance with claim 1, wherein each of the twoorthogonal codes are based on [1,1] or [1,−1], if the number of layersis less than or equal to the predetermined number of layers.
 7. Asubscriber station, comprising: a downlink receive path comprisingcircuitry configured to receive at least one reference signal in one ormore subframes, each subframe comprising two slots, each slot comprisingS OFDM symbols, each of the S OFDM symbols comprising N resourceelements; and a reference signal controller configured to: apply oneorthogonal code of two orthogonal codes to a antenna port of a first setantenna ports, if the number of layers is less than or equal to apredetermined number of layers, and apply one orthogonal code of fourorthogonal codes to a antenna port of a second set antenna ports, if thenumber of layers is greater than the predetermined number of layers,wherein total number of antenna ports is based on the number of layers.8. A subscriber station in accordance with claim 7, wherein one antennaport comprises 4 resource elements.
 9. A subscriber station inaccordance with claim 7, wherein resource elements for the at least oneantenna port are placed on 6th and 7th OFDM symbols in the each slot.10. A subscriber station in accordance with claim 7, wherein a firstantenna port within the first set is mapped on a first 4 resourceelements with a first orthogonal code, a second antenna port within thefirst set is mapped on a first 4 resource elements with a secondorthogonal code, a third antenna port within the first set is mapped ona second 4 resource elements with the first orthogonal code, and afourth antenna port within the first set is mapped on a second 4resource elements with the second orthogonal code.
 11. A subscriberstation in accordance with claim 7, wherein the predetermined number oflayers is
 4. 12. A subscriber station in accordance with claim 7,wherein each of the two orthogonal codes are based on [1,1] or [1,−1],if the number of layers is less than or equal to the predeterminednumber of layers.
 13. A method for transmission of reference signal, themethod comprising: applying one orthogonal code of two orthogonal codesto an antenna port of a first set antenna ports, if the number of layersis less than or equal to predetermined number of layers; applying oneorthogonal code of four orthogonal codes to an antenna port of a secondset antenna ports, if the number of layers is greater than thepredetermined number of layers; transmitting at least one referencesignal in one or more subframes, each subframe comprising two slots,each slot comprising S OFDM symbols, each of the S OFDM symbolscomprising N resource elements, wherein total number of antenna ports isbased on the number of layers.
 14. The method of claim 13, wherein oneantenna port comprises 4 resource elements.
 15. The method of claim 13,wherein resource elements for the at least one antenna port are placedon 6th and 7th OFDM symbols in the each slot.
 16. The method of claim13, wherein a first antenna port within the first set is mapped on afirst 4 resource elements with a first orthogonal code, a second antennaport within the first set is mapped on a first 4 resource elements witha second orthogonal code, a third antenna port within the first set ismapped on a second 4 resource elements with the first orthogonal code,and a fourth antenna port within the first set is mapped on a second 4resource elements with the second orthogonal code.
 17. The method ofclaim 13, wherein the predetermined number of layers is
 4. 18. Themethod of claim 13, wherein each of the two orthogonal codes are basedon [1,1] or [1,−1], if the number of layers is less than or equal to thepredetermined number of layers.
 19. A method for reception of referencesignal, comprising: applying one orthogonal code of two orthogonal codesto an antenna port of a first set antenna ports, if the number of layersis less than or equal to predetermined number of layers; applying oneorthogonal code of four orthogonal codes to an antenna port of a secondset antenna ports, if the number of layers is greater than thepredetermined number of layers; receiving at least one reference signalin one or more subframes, each subframe comprising two slots, each slotcomprising S OFDM symbols, each of the S OFDM symbols comprising Nresource elements, wherein total number of antenna ports is based on thenumber of layers.
 20. The method of claim 19, wherein one antenna portcomprises 4 resource elements.
 21. The method of claim 19, whereinresource elements for the at least one antenna port are placed on 6thand 7th OFDM symbols in the each slot.
 22. The method of claim 19,wherein a first antenna port within the first set is mapped on a first 4resource elements with a first orthogonal code, a second antenna portwithin the first set is mapped on a first 4 resource elements with asecond orthogonal code, a third antenna port within the first set ismapped on a second 4 resource elements with the first orthogonal code,and a fourth antenna port within the first set is mapped on a second 4resource elements with the second orthogonal code.
 23. The method ofclaim 19, wherein the predetermined number of layers is
 4. 24. Themethod of claim 19, wherein each of the two orthogonal codes are basedon [1,1] or [1,−1], if the number of layers is less than or equal to thepredetermined number of layers.